Hydraulic fracture growth is simulated in homogeneous granite with a fully coupled hydromechanical discreteelement method. Three models are considered, namely an intact rock and two models with pre-existing fractures, connected or disconnected to the incipient hydraulic fracture plane. In all scenarios, the hydraulic fracture grows in a plane with a mostly circular front, despite anisotropic stresses. In the intact rock and disconnected models a hydraulic fracture propagates normal to the minimum principal stress, whereas in the connected fracture model fluid leaks into a pre-existing low dipping fracture set. The length of the hydraulic fracture increases with the square root of time and the maximum aperture is linearly correlated to the length of the fracture, except for the connected fracture model, where the aperture is limited and mostly constant over time. These results demonstrate the important role pre-existing natural fractures can have on hydraulic fracture growth. Stress perturbation due to the developing hydraulic fracture promotes fracturing at the fracture tip but inhibits failure in the fracture walls. In the connected fracture model these stress perturbations are significantly reduced, due to fluids moving into the natural fractures.
Hydraulic fractures are fluid-induced and connected hydraulically to the injection well. They form a more or less complex geometry with an architecture ranging from a simple planar structure to a complex fractured zone [3, 4, 5, 6]. The volume of the hydraulic fractures commonly increases over the course of the fluid injection indicating fracture propagation and linkage . Hydraulic fractures commonly grow normal to the minimal principal stress [8, 9]. It is therefore interpreted as resulting of tensile opening due to increased pore pressure at fracture tips and in the fracture volume [10, 11, 12]. However, evidence of shearing has also been highlighted [3, 8, 13, 14].